The present invention relates to an image forming apparatus such as a copying machine which forms a toner image by attracting toner powder onto photosensitive means and then transfers the toner image onto a sheet of copy paper. More particularly, the invention relates to an image forming apparatus capable of properly adjusting image density distribution in a main scanning direction of the copy paper.
Generally in image forming apparatuses such as copying machines, a photosensitive drum is charged to a certain potential by charging means and exposed to light to produce an electrostatic latent image of an original document. Distribution of charge potential on the surface of the photosensitive drum is usually flattened in its axial direction in order to create uniform image density on the copy paper in its main scanning direction (or widthwise).
Even when the charge potential distribution in the axial direction of the photosensitive drum is flat, however, the latent image density distribution in the same direction may become uneven due to variations in exposure to light from a lamp, a disparity in the charging efficiency of the photosensitive drum, or smears on a charge wire or a shield case that constitute the charging means. In such a case, density of the toner image produced on the surface of the photosensitive drum becomes uneven in the main scanning direction, causing partial fog on the front or rear side, for example, when the toner image is transferred onto the copy paper.
Various arrangements have been proposed hitherto to solve this problem.
One example of such arrangements is disclosed in Japanese Unexamined Patent Publication No. 61-151669. According to the disclosure, there is made provision for measuring surface potential of the photosensitive drum, for example. Then, based on measurement results, the surface potential distribution is evened out by adjusting the tilt angle formed by the charge wire and the surface of the photosensitive drum in its axial direction.
Another example is disclosed in Japanese Unexamined Patent Publication No. 3-17667. According to this disclosure, there are provided on the downstream side of the photosensitive drum a plurality of density sensor elements arranged at regular intervals in the main scanning direction. As the copy paper is separated from the photosensitive drum and conveyed downstream, the density sensor elements detect image density on the copy paper. Then, exposure of the photosensitive drum is controlled based on the resultant image density data.
In the first example of known arrangements described above, the tilt angle of the charge wire is adjusted based on measurements of surface potential of the photosensitive drum. With this process, however, it is essentially difficult to completely eliminate the cause of fog. This is because unevenness on the copy paper in its main scanning direction cannot be fully suppressed in case the image density unevenness results from irregularities in optical system's luminous intensity, sensitivity of the photosensitive drum or moisture absorption status of the copy paper.
In the second example of known arrangements described above, it is difficult to obtain reliable image density data. This is because the distance between the density sensor elements and copy paper can hardly be kept constant since image density is measured while the copy paper is being conveyed. Another problem of this arrangement is that because the density sensor elements are placed at intervals in the main scanning direction of the copy paper, there exist gaps or dead zones between individual sensor elements, making it difficult to obtain an accurate measurement of the image density distribution. Moreover, the use of a plurality of density sensor elements is likely to result in a higher product cost.